A monochromator includes an entrance slit plate having an entrance slit, a light separator (such as a diffraction grating, a prism, etc.), and an exit slit plate having an exit slit. Light coming into a monochromator through the entrance slit is separated (or dispersed) by the light separator into a series of component monochromatic lights, and an image of the entrance slit of every component monochromatic light is projected onto the exit slit plate. Thus a monochromated light of varying wavelength comes out of the exit slit while the monochromator scans through a preset range of wavelength.
In conventional monochromators, the entrance and exit slits are both rectangular long in the direction perpendicular to the direction of the separation of light (the direction of the separation of light is hereinafter referred to as the lateral direction). A spectrophotometer that uses a monochromator equipped with slits of such shape has the following problem.
When a sample is measured in a spectrophotometer, a bundle of light coming out of the exit slit of the monochromator is focused onto the sample where an image of the exit slit is formed, as shown in FIG. 1B. Thus the bundle of light 11 for measuring the sample 12 is shaped rectangular bearing the shape of the exit slit. When an ordinary box sample cell is used with an enough amount of sample in it, it is possible to set all the measurement light pass through the sample. When a flow sample cell is used or when a small-sized sample or small amount of sample is measured, however, it occurs that only a part of the measurement light can pass through the sample. Thus the efficiency of light in the measurement becomes low, since, as shown in FIG. 1B, the part 13 of the measurement light 11 external of the sample 12 is not used for the measurement.